From the prior art, hybrid vehicles that comprise a hybrid transmission are known. In addition to the internal combustion engine, they comprise an electric motor or electric machine. In serial hybrid vehicles, a generator is powered by the internal combustion engine and the generator supplies electrical energy to the electric motor, which drives the wheels. Furthermore, parallel hybrid vehicles are known, in which a summation of the torques of the internal combustion engine and at least one electric machine that can be connected with the internal combustion engine takes place. In such cases, the electric machines can be connected to the belt drive or to the crankshaft of the internal combustion engine. The torques produced by the internal combustion engine and/or the at least one electric machine are transmitted to the driven axle by a downstream transmission unit.
From the prior art, in the case of parallel hybrid vehicles, it is known to couple the electric machine not directly between the internal combustion engine and the transmission, but rather via a single-stage planetary gear set in a releasable manner between the combustion engine and the transmission, whereby an electro-dynamic drive system is formed.
For example, U.S. Pat. No. 6,805,648 B1 by the present Applicant, the content of the disclosure relating to the electrodynamic drive system is incorporated herein by reference and describes an electrodynamic drive system for a vehicle which comprises, between a drive engine and a main transmission; a planetary gear set that serves as a summation gear system and includes the three elements: a sun gear, a ring gear and a planetary gear carrier. In this, a first element of the planetary gear set is connected to the main transmission, a second element to the drive engine and a third element to at least one electric motor so that a wear-free starting element for the vehicle is formed.
DE 101 52 471 A1 by the present Applicant describes a method for starting an internal combustion engine of a vehicle equipped with an electrodynamic drive system, which comprises an electric machine and a planetary gear set between the internal combustion engine and a main transmission with an input shaft.
In this case, the ring gear of the planetary gear set is connected in a rotationally fixed manner to the crankshaft of the internal combustion engine via a damping device and the carrier of the planetary gear set can be connected releasably to the rotor of the electric machine, which is connected in a rotationally fixed manner with the sun gear of the planetary gear set, by way of a friction clutch. The friction clutch serves as a bridging clutch of the planetary gear set, since when the friction clutch is engaged the planetary gear set is locked.
In addition, the carrier of the planetary gear set is connected to the input shaft of the main transmission and the input shaft can be releasably coupled via a brake to a transmission housing.
Even in vehicles which comprise an electrodynamic drive system, the vehicle often has to be braked for a longer time. When this is done using the normal service brakes, there is a disadvantageous risk that the brakes will overheat and reduce the braking action. A possibility for avoiding this is to use wear-free brakes, such as retarders, but this demands more structural space and results in increased production and assembly costs.
During the normal driving operation of vehicles having an electromechanical drive system, the bridging clutch is engaged so that the planetary gear set of the electrodynamic drive system is locked. Since braking is part of normal driving operation, in such a situation, a braking torque is produced by the combustion engine, the electric machine (respectively at the same speed) or by an additional, built-in retarder.
The purpose of the present invention is to indicate a method for wear-free braking in a vehicle having an electrodynamic drive system with which the need for an additional, wear-free brake such as a retarder is eliminated.